In general, a semiconductor chip package or module having leads is connected to a chip carrier substrate by a bonding material such as solder. Various lead connection testing methods have been proposed, including an optical testing method using a camera, an X-ray testing method, and an electric testing method in which a conduction test is performed. However, the detection accuracy of these testing methods is limited. For example, as shown in FIG. 9, solder joints or bond portions 14, which connects leads 12 of a chip package 10 to conductors 16 of a chip carrier substrate 18 such as a printed circuit board or a ceramic substrate, may include a crack 24 or foreign matter 26. Since the connection strength of such a lead is essentially low even though it may have a good appearance, after shipment of a product it may peel off from the substrate due to some external force or the like, causing an open circuit fault. Recently, the number of leads per chip has greatly increased, and the solder amount per lead has been reduced accordingly. Therefore, inclusion of even a small crack or scratch or a small foreign body may break the connection, causing a conduction fault or an open circuit fault. However, the optical testing method, X-ray testing method and electric testing method cannot find all such minute defects within solder joints, which are potential causes of conduction faults.
Japanese Published Examined Patent Application No. 61-7016 discloses a technique of testing the quality of lead connections by applying a vertical vibration to a selected point of the top surface of a lead from a vibrator probe and detecting resulting variation in the probe vibration with a piezoelectric pickup. Although this method can detect a connection defect of a kind which cannot be detected by the optical testing method and so on, it has the problem of slow detection speed. More specifically, even a lead connection with the defect of insufficient solder may be detected as a good connection if solder happens to exist at the point of probe landing. To avoid this problem, it is necessary to use a plurality of testing points for each lead, in which case the probe is moved to a plurality of testing points and the testing procedure is repeated for each of the testing points. Therefore, if there are a large number of leads, the testing speed is very slow. In the method of this patent, the connection quality is judged by analyzing the vibration detected by the piezoelectric pickup, probe tapping sound detected by a microphone, or lead surface displacement detected optically.
Japanese Published Unexamined Patent Application No. 2-54169 discloses a technique of testing connection quality by contacting a vibration finger with a lead of an IC package and detecting the vibration of the lead with a pickup. In this method, the vibration frequency of the finger is changed, and the frequency-vibration characteristic of the lead is detected. The vibration finger is inserted into a gap between adjacent leads at a position close to the package, and brought in contact with the side of the lead. Since the vibration is applied to the lead at the position away from the actual solder joint, there exists a problem of a small difference between signals associated with good and defective connections, i.e., a low defect discrimination ability. Further, since the finger needs to be inserted into a gap between the leads, this method cannot be applied to a high-density package with a small lead pitch. In the method of this patent publication, the connection quality is judged by comparing the frequency-vibration characteristics of the pickup output with reference characteristics, which requires complex processing.